Viscosity increase due to soot accumulation in the oil can cause severe damage to diesel engines as a result of slow drainage of oil back to the sump, gelation of the oil in the sump, or filter plugging, all of which can result in reduced oil flow to critical bearings and oil-cooled parts, such as pistons.
Rust inhibition is also a key performance parameter for lubricating oils. Poor rust inhibiting properties can result in stuck valve lifters, impairing the operation of the engine.
Sulfurized, carbonated, calcium alkylphenates are well known detergents for lubricating oils, including heavy duty diesel crankcase applications. They provide certain advantages over other types of metallic detergents, specifically sulfonates, in that they impart oxidation protection to the lubricating oil. Unfortunately, formulations based solely on sulfurized, carbonated, calcium alkylphenates as the overbased detergent are deficient in rust inhibition, and are usually formulated with special rust inhibitors. These inhibitors add cost to the formulation and can sometimes cause compatibility problems with the other additives in the lubricating oil formulation.
We have discovered that a certain combination of a carbonated sulfurized metal alkyl phenate and a carbonated metal alkyl aryl sulfonate gives both good soot dispersancy and good rust inhibition.
U.S. Pat. No. 3,236,770 teaches a diesel lubricating oil with basic sulfonates or basic phenates, in combination with a dialkylthiophosphate. Sulfonates and phenates are used as alternatives, not mixtures.
U.S. Pat. No. 4,328,111 teaches a diesel lubricating oil with the reaction product of a basic compound (overbased metal sulfonate, phenate, or mixture thereof) with a phosphorus-containing material. The mixture of phonate and sulfonate is not discussed in the specification and is not found in the examples.
U.S. Pat. No. 4,938,881 teaches a diesel lubricating oil with a carboxylic dispersant, a salt of sulfonic or carboxylic acid, a dithiophosphoric acid, and a carboxylic ester derivative. It can also contain a salt of a sulfur acid, phosphorus acid, carboxylic acid, phenol, or mixtures thereof. The preferred salt is of an alkylated benzene sulfonic acid.
U.S. Pat. No. 5,071,576 teaches a mixture of overbased phenate and overbased alkyl aryl sulfonate. The sulfonate has at least one long chain alkyl group (greater than 40 carbon atoms). Up to 85% of the base can be contributed by the phenate.
U.S. Pat. No. 5,202,036, which is similar to U.S. Pat. No. 4,938,881, teaches a diesel lubricating oil with a carboxylic dispersant and a salt of acidic organic compound. The salt can be of sulfur acid, phosphorus acid, carboxylic acid, phenol, or mixtures thereof. The preferred salt is of an alkylated benzene sulfonic acid.
EP Patent Application 552892-A1 teaches a diesel lubricating oil with zinc dithiophosphate, an antioxidant, an overbased metal sulfurized phenate, and a metal alkyl aromatic sulfonate. Preferably, the phenate constitutes from 50 wt. % to 80 wt. % of the phenate/sulfonate, but the sulfonate is not overbased.